1. Field of the Invention
The present invention relates to a substrate voltage(Back Bias Voltage) detection control circuit, and in particular to an improved substrate voltage detection control circuit which is capable of separately operating substrate voltage detectors in accordance with an operation mode of chips.
2. Description of the Background Art
Generally, a substrate VBB is an important parameter for implementing a stable operation of a DRAM chip. For example, the substrate voltage VBB prevents a data loss of a memory cell or a latch-up by preventing a PN junction in a DRAM chip from being forward biased. In addition, the substrate voltage VBB decreases a threshold voltage variation of a MOS transistor based on a body effect and implements a stable circuit operation.
The substrate voltage VBB is generated by a substrate voltage generation circuit of the chip. The substrate voltage generation circuit generates a substrate voltage VBB of -2V through -3V based on a power voltage VCC of 5V and provides it to a P-well formed on a substrate.
FIG. 1 illustrates a known substrate voltage generation circuit which is formed of an oscillator 10, a charge pump 20, and a substrate voltage detector 30.
The oscillator 10 is a ring oscillator formed of an odd number of inverters. The charge pump 20 may be formed of two NMOS transistors connected with a diode, respectively. In addition, the substrate voltage detector 30 may be formed of a NMOS transistor having its gate grounded. The threshold voltage of the NMOS transistor becomes a detection level of the substrate voltage VBB.
The operation of the known substrate voltage generation circuit will be explained.
The charge pump 20 performs a pumping operation in accordance with an oscillation signal OSC from the oscillator 10 and outputs a substrate VBB, and the substrate voltage detector 30 compares the substrate voltage VBB from the charge pump 20 with a previously set detection level and outputs a detection signal DET.
The oscillator 10 drives the charge pump 20 when a high level detection signal DET is inputted from the substrate voltage detector 30 (if the substrate voltage is higher than the detection level as a result of the judgement) and then supplies a substrate voltage VBB. When a low level detection signal DET is inputted (when the substrate voltage is smaller than the detection level), the charge pump 20 is stopped.
The level of the substrate voltage VBB has a negative value by the electron supplied from the charge pump 20 by repeatedly performing the above-described operation, so that the substrate voltage VBB maintains a predetermined level.
Generally, the substrate voltage VBB is determined by a balance between the current(generated by electrons) of the substrate voltage generation circuit and the substrate current (generated by positive hole) Ibb. At this time, the source of the substrate Ibb is the NMOS transistor of the chip. The channel electrons are accelerated by a high electric field at the peripheral portions of the NMOS transistor for thereby generating an impact ionization phenomenon. As a result, the pairs of electron-positive holes are generated by the impact ionization. The thusly generated positive holes are introduced into the substrate for thereby forming a substrate voltage, so that the substrate voltage is increased in the + direction.
Therefore, when the chip is in the standby mode, since the NMOS transistor is not operated, the quantity the positive holes introduced into the substrate is small, and when the chip is in the active mode, a plurality of NMOS transistors in the chip are operated for thereby increasing the quantity of the positive holes. When the power voltage VCC is increased, the substrate current Ibb is sharply exponentially increased. In this case, since the period of the ring oscillator forming the oscillator 10 is one-dimensionally increased, when the power voltage VCC exceeds a predetermined level, the substrate current Ibb introduced into the substrate exceeds the current supply capacity of the substrate voltage generation circuit. Namely, the number of the positive holes introduced into the substrate by the impact ionization exceeds the number of the electrons supplied from the substrate voltage generation circuit to the substrate, so that the substrate voltage VBB is increased in the + direction.
When the substrate voltage VBB is increased in the + direction by the substrate current Ibb, the substrate voltage detector 30 detects the level of the substrate voltage VBB and operates the oscillator 10 and the charge pump 20 sequentially. As a result, the number of the electrons supplied to the substrate is increased, and the substrate voltage VBB is uniformly maintained. In addition, the above-described procedures are repeatedly performed during the operation of the chip, and the period of the repetition is determined by the amount of the current (substrate current) introduced into the substrate.
Therefore, in order to overcome the problem that the substrate voltage is increased by the substrate current, a method for increasing the driving capacity of the substrate voltage generation circuit is disclosed. However, in this case, the driving capacity in the active mode of the chip is also increased, so that the consumption of the current is increased.
In addition, in the substrate voltage generation circuit, the substrate voltage detector is always turned on, even when the reflash is performed, the current is used up like the standby mode.
When turning on the substrate voltage detector at every predetermined period (few .mu.s) for decreasing the current consumption in the standby mode, it is impossible to sensitively react to the variation of the substrate voltage in the active mode. In addition, the charge pump is driven at every predetermined period(few .mu.s) by the substrate voltage detector, so that the level of the substrate voltage is increased more than the set value.